T&T: Pontoon boat question, was Low boat speed = low power.

[Gary Bell]

In the example of the good Captain's slow pontoon boat we can safely 
ignore planing behavior (given the lack of flat planing surfaces and the 
low speed), non linear changes in skin friction at different speeds, 
changes in waterline length at different hull speeds and a bunch of 
other complexifiers because they only contribute in small ways. 

All hulls that are afloat and at zero speed are entirely displacement 
hulls, that is they float because they displace an amount of water equal 
to their weight  -- and entirely regardless of their shape.  Add more 
weight and the hull sinks more deeply into the water until it displaces 
enough more water to account for the increased weight -- so draft is 
proportional to weight.  When that hull is moved slowly through the 
water energy is required to shove aside the water that was in front of 
the hull and draw back in the water behind, but the amount of water 
shoved aside is not proportional to the weight or the total displacement 
but instead is proportional to the frontal cross sectional area (how 
deep and wide the hull appears to be when viewed from ahead, or the area 
of its frontal silhouette).  A hull which is deeper in the water will 
have a greater frontal area and will have to shove aside more water to 
proceed.  In the case of a pontoon boat (or my beloved slender hulled 
power catamaran), the long slender hulls get lots of extra displacement 
for a very small increase in frontal area.  Thus resistance to forward 
movement will not increase as much as with a beamy short hull.   In this 
case increasing the weight indeed increased the draft, but the frontal 
area was increased only a tiny proportion because the hulls are so long 
and slender.  I expect that his final speed was indeed slightly slower 
with the heavy load, but likely not enough diminished to be noticed. 

Increasing the weight of the boat increases its inertia as well.  I 
gather that the increase in weight of the additional passengers 
comprising a "full boat" is substantial, and in our four knot example it 
seems to me that the increase in inertia accounts for the noted increase 
in time required to accelerate. 

The old drone again,
Mister Science